ΔG represents the difference in free energy state between the products and the reactants in a reaction. Systems favor low-energy states. Thus, a reaction proceeds in the direction that decreases the system's free energy.
Enzymes do not affect ΔG. Therefore, they do not affect the direction, extent, or spontaneity of a reaction.
If the reaction, A + B → C + D, is characterized by ΔG = −3.0 kJ/mol, then by definition, the reverse reaction, C+D → A + B, is characterized by ΔG = +3.0 kJ/mol.
In this example, A + B → C + D is said to be exergonic. It proceeds spontaneously (i.e., no energy input is required to drive the reaction).
The reverse reaction, C + D → A + B is said to be endergonic. It does not proceed spontaneously but will occur if sufficient energy is added to the system.
The equilibrium constant
For the reaction, A + B ←→ C + D. The equilibrium constant is
K′eq = [C] [D] / [A]
K′eq represents the ratio of the concentration of products to reactants when the reaction is at equilibrium (the rates of the forward and reverse reactions are equal, and there is no net change in the amounts of products or reactants).
When K′eq > 1, the equilibrium lies to the right, and favors formation of products.
When K′eq < 1, the equilibrium lies to the left and favors formation of reactants.
ΔG and K′eq are related by the expression:
ΔG = ΔGo′ + RT ln (K′eq)
ΔGo′ represents the standard free-energy change, or the change in free energy when the concentration of the reactants and products are each 1.0 M and the pH is 7. R is the gas constant (8.31 J/mol/K, but do not memorize it), and T is the absolute temperature.
By the way this equation is not high yield and is unlikely to be tested in USMLE.
And here's the table that will let you answer all questions: click to enlarge
Yes, you'll find it in the biochemistry book in the Enzyme kinetics chapter as far as I remember.
But Kaplan did not explain it in a nice way.
I don't think you need to know more than what I just posted for the purpose of the USMLE exam.
As for practice questions, you'll face them when you do your Kaplan Qbank and UWorld.